# The Milky Way. Mass of the Galaxy, Part 2. Mass of the Galaxy, Part 1. Phys1403 Stars and Galaxies Instructor: Dr. Goderya

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1 Foundations Chapter of Astronomy 15 13e Our Milky Way Seeds Phys1403 Stars and Galaxies Instructor: Dr. Goderya Selected Topics in Chapter 15 A view our Milky Way? The Size of our Milky Way The Mass of our Milky Way Structure of our Milky Way Two types of Clusters Two types of Stars The Nucleus of the Milky Way Supper Massive Black Hole in our Galaxy The Milky Way Cepheid Variable Stars Used as Beacons to determine the Size of Galaxy Almost everything we see in the night sky belongs to the Milky Way We see most of the Milky Way as a faint band of light across the sky From the outside, our Milky Way might look very much like our cosmic neighbor, the Andromeda galaxy Cepheids could be used to determine distances if their true luminosities could be discovered The more distant a star is, the smaller its proper motion tends to be Proper motions contain clues to distance. Magnitude-distance formula Mass of the Galaxy, Part 1 Mass of the Galaxy, Part 2 To find the mass of an object, astronomers must observe the motion of another object orbiting it Total mass M of a binary star system equals the cube of the average separation of the stars a divided by the square of the period P Stars at different distances from the center revolve around the center of the galaxy with different periods stars starting near each other draw apart as time passes Rotation Curve of our Galaxy Orbital velocities do not decline outside the orbit of the Sun contrary to Kepler s 3 rd Law. Flat portion of curve. There must be some other mass that must be causing this to happen. Dark Matter exist in the Galaxy 1

2 The Structure of the Milky Way (1) Exploring the Galaxy Using Clusters of Stars Two types of star clusters: 1) Open clusters: young clusters of recently formed stars; within the disk of the Galaxy Open clusters h and Persei Disk Nuclear Bulge Halo Globular Clusters 2) Globular clusters: old, centrally concentrated clusters of stars; mostly in a halo around the Globular Cluster M 19 Galaxy Globular Clusters Dense clusters of 50,000 1 million stars Old (~ 11 billion years), lower-mainsequence stars Approx. 200 globular clusters in our Milky Way Globular Cluster M80 Locating the Center of the Milky Way Distribution of globular clusters is not centered on the sun but on a location which is heavily obscured from direct (visual) observation Stellar Populations Population I: Young stars: metal rich; located in spiral arms and disk Population II: Old stars: metal poor; located in the halo (globular clusters) and nuclear bulge Metals in Stars Absorption lines almost exclusively from hydrogen: Population II Many absorption lines also from heavier elements (metals): Population I At the time of formation, the gases forming the Milky Way consisted exclusively of hydrogen and helium. Heavier elements ( metals ) were later only produced in stars. => Young stars contain more metals than older stars 2

3 The Nucleus of the Galaxy At visual wavelengths, this region is totally hidden by interstellar dust Infrared and radio wavelengths can show a region of tremendously crowded stars orbiting in the nucleus at high velocities along with complicated structures of atomic and molecular gas clouds Radio View of the Galactic Center Complex structures caused by magnetic fields and star formation Nucleus is crowded Supermassive black hole in the center Chapter 16 Galaxies Foundations of Astronomy 13e Seeds Selected Topics in Chapter 16 What is a Galaxy? Diversity and Types of Galaxies Elliptical and Spirals Distances to Galaxies Diameter and Luminosity Mass Dark Matter Detecting Dark Matter via Gravitational Lensing Cluster of Galaxies Local group What is a Galaxy? How Many Galaxies Are There? Star systems like our Milky Way Contain a few thousand to tens of billions of stars. Probably 200 billion Large variety of shapes and sizes 3

4 The Shapes of Galaxies Types of Galaxies Elliptical galaxies have no disk, no spiral arms, and almost no gas and dust Range from huge giants to small dwarfs Spiral galaxies: disk-shaped Typically have spiral arms and contain gas and dust Variations: barred spiral and lenticular galaxies Irregular galaxies: generally shapeless and tend to be rich in gas and dust futurism.com Hubble Tuning Fork Diagram Measuring the Properties of Galaxies Must first determine distance in order to calculate diameter, luminosities and mass of a galaxy Las Cumbres Observatory Distance Measuring Distance 1. Find a standard candle in a galaxy 2. Measure its apparent brightness 3. Calculate its distance Cepheid Method Measure Cepheid s period, find its luminosity, compare to apparent magnitude, find distance Type Ia Supernovae Have standard luminosities, compare to apparent magnitude, find distance 4

5 The Most Distance Galaxies At very large distances, only general characteristics of galaxies can be used to estimate luminosities, thus estimating distances Astronomers use a distance scale The look-back time of these distant galaxies becomes an appreciable fraction of the age of the Universe The Hubble Law Distant galaxies are moving away from our Milky Way, with a recession velocity (V), proportional to their distance (d): (where H is the Hubble constant = 70 Km/s/Mpc) Diameter and Luminosity Galaxies differ dramatically in size and luminosity Irregular galaxies: small with low luminosity Most spiral galaxies: large with high luminosities Elliptical galaxies: a wide range of diameters and luminosities Mass of Galaxies From Rotation Curves Measuring mass reveals two things: Range of masses is wide Galaxies contain invisible dark matter spread through extended galactic coronae Measured masses are about ten times larger than the masses that can be seen Nearly all galaxies contain dark matter Gravitational Lensing: Detecting Dark Matter Mass bends the light of a much more distant galaxy to produce arcs that are actually distorted images of the distant galaxy Detecting Dark matter in other Galaxies Two clusters colliding Pink color: normal matter Purple color indicates dark matter Clusters of Galaxies Galaxies generally do not exist in isolation, but form larger clusters of galaxies Rich clusters 1,000 or more galaxies, diameter of ~ 3 Mpc, condensed around a large, central galaxy Poor clusters Less than 1,000 galaxies (often just a few), diameter of a few Mpc, generally not condensed towards the center 5

6 Our Galaxy Cluster: The Local Group, Part 1 Our Galaxy Cluster: The Local Group, Part 2 Some galaxies lie in the plane of the Milky Way Galaxy and are difficult to detect Acknowledgment The slides in this lecture is for Tarleton: PHYS1411/PHYS1403 class use only Images and text material have been borrowed from various sources with appropriate citations in the slides, including PowerPoint slides from Seeds/Backman text that has been adopted for class. 6

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